1. Technical Field
The invention relates to communications systems. More particularly, the invention relates to a channel assignment scheme for cellular mobile communications systems employing multi-beam antennas with beam selection.
2. Description of the Prior Art
A cellular network allows mobile communications in a specified geographic area by dividing the area into sectors, each of which is serviced by a cell site dedicated to that particular sector. Cellular systems work because a low power mobile unit is passed from cell site to cell site as the unit moves about in the service area from sector to sector. During the passing off, the mobile unit may be assigned a different communication frequency, known as a channel, corresponding to the channels allocated to the cell site in the mobile unit's current sector. In this way, the frequency spectrum within a limited service area is reused. Thus, each channel within the system may be used at a plurality of cell sites simultaneously and the system can therefore support a number of users far in excess of channels and frequency spectrum otherwise available.
There are two efficiencies that express performance of cellular communications systems: 1) power efficiency, and 2) spectrum efficiency (see, for example M. Hata, K. Kinoshita, K. Hirade, Radio Link Design of Cellular Land Mobile Communication Systems, IEEE Trans. VT., vol. VT-31, pp. 25-31, 1982). Power efficiency indicates how efficiently the transmitted power can be used for communications. With regard to power efficiency, a better design of the radio access scheme requires less received signal strength. Spectrum efficiency corresponds to the system user capacity for mobile communications systems that have a cellular configuration, where the same frequency spectrum is reused in geographically separated cells. A key concern with regard to spectrum efficiency in cellular mobile communications systems is resistance to co-channel interference (see, for example M. Mouly, M. B. Pautet, The GSM System for Mobile Communications, published by the authors: 4, rue Louise Bruneau, F-91120 Palaiseau, France, pp. 601-611, 1992).
Various schemes have been proposed for the efficient reuse and handling of channels for cellular communications systems. For example, R. Alexis, Method and Apparatus For Selecting A Free Channel In A Mobile Radio System, U.S. Pat. No. 4,783,780, 8 Nov. 1988 (a mobile radio system in which power levels in a signaling path are measured and compared by a mobile station with an interference threshold power level to identify duplex channels of interfering fixed stations, such that a free duplex channel is selected by the mobile station by excluding channels of interfering and engaged fixed stations); V. Graziano, Antenna Array For A Cellular RF Communications System, U.S. Pat. No. 4,128,740, 5 Dec. 1978 (an array of antennas for a cellular RF communications system consisting of a plurality of antenna sites at which a plurality of sectored antennas provide a plurality of communications channels, and in which channels are allocated to optimize spectrum use, while minimizing interference); and D. Reudink, Y. Yeh, Technique For Efficient Spectrum Utilization In Mobile Radio Systems Using Space Diversity, U.S. Pat. No. 4,355,411, 19 Oct. 1982 (a mobile radio system in which a mobile station is operated on a first communication channel when the mobile station is not experiencing interference, and in which the mobile station is switched to a different communications channel when an interference threshold level is exceeded).
The use of a combined directional multibeam antenna with beam selection is known to be effective in reducing the amount of mobile station transmitter power required to establish and maintain an acceptable communications path. A antenna beam exhibits sensitivity to signals that are received from a specific direction. In such scheme an array of beams is employed, and a beam having the largest received signal strength is selected. Hence, the use of a multibeam antenna system with beam selection improves power efficiency in a communications system.
However, with regard to co-channel interference, the beam selector may select a beam that is subject to co-channel interference if such beam produces the largest received signal strength. Such interfering selection is more likely to happen as the cell radius in a cellular communications system becomes smaller.
The problem of co-channel interference can be eliminated by decision oriented channel estimation schemes (see, for example M. Schwartz, W. R. Benett, S. Stein, Communication Systems and Techniques, McGraw-Hill, pp. 490-584, 1966). In such schemes, an adaptive filter based upon the least-square error criterion is used for channel estimation, where the Euclidean distance between the desired and the received signals is used as an error component. A signal having the smallest squared error is then selected from among the signals received by the multiple beams.
Another simple beam selection scheme is based upon unique word detection, where a unique word or pilot signal is embedded in a transmitted symbol stream. The receiver correlates the received signal with the unique word pattern. A signal having the largest correlation value is then selected.
It is well understood in the art that a multibeam antenna system that employs one of the various known beam selection schemes can improve the spectrum efficiency. This is because it is unlikely that the interference source is located coincidentally in the pattern of the desired beam. However, if cellular system communications frequencies are allocated to each beam in the multibeam antenna system, such that the spectrum reuse distance between each beam within the sector satisfies the system's signal-to-interference power ratio ("SIR") requirement, then such a multibeam antenna system with beam selection is equivalent to a sectored cell configuration. Accordingly, no advantageous outcome over the sectored cell would result from beam selection.
It would be advantageous to provide an improved beam selection scheme for a multibeam antenna system, such that beam selection improves power and spectrum efficiencies in a cellular communications system.